fix signature provider crash on protocol callbacks

Author: lukaszsamson

apps/language_server/lib/language_server/providers/signature_help/signature.ex

@@ -138,15 +138,23 @@ defmodule ElixirLS.LanguageServer.Providers.SignatureHelp.Signature do
                     }
 
                   nil ->
-                    fun_info = Map.fetch!(metadata.mods_funs_to_positions, {mod, fun, arity})
-
-                    {spec, doc, _} =
-                      Metadata.get_doc_spec_from_behaviour(
-                        behaviour,
-                        fun,
-                        arity,
+                    category = case metadata.mods_funs_to_positions[{mod, fun, arity}] do
+                      nil ->
+                        if Code.ensure_loaded?(mod) and macro_exported?(mod, fun, arity) do
+                            :macro
+                        else
+                          :function
+                        end
+                      %State.ModFunInfo{} = fun_info ->
                         State.ModFunInfo.get_category(fun_info)
-                      )
+                    end
+
+                    {spec, doc, _} = Metadata.get_doc_spec_from_behaviour(
+                      behaviour,
+                      fun,
+                      arity,
+                      category
+                    )
 
                     %{
                       signature

apps/language_server/test/providers/signature_help/signature_test.exs

@@ -1292,6 +1292,32 @@ defmodule ElixirLS.LanguageServer.Providers.SignatureHelp.SignatureTest do
       end
     end
 
+    test "retrieve metadata function signature - fallback to remote protocol callback" do
+      code = """
+      defimpl Enumerable, for: Date do
+        def count(a) do
+          :ok
+        end
+      end
+
+      Enumerable.impl_for()
+      """
+
+      res = Signature.signature(code, 7, 21)
+
+      assert %{
+               active_param: 0,
+               signatures: [
+                 %{
+                   documentation: "A function available in all protocol definitions" <> _,
+                   name: "impl_for",
+                   params: ["data"],
+                   spec: "@callback impl_for(term()) :: module() | nil"
+                 }
+               ]
+             } = res
+    end
+
     test "retrieve metadata macro signature - fallback to macrocallback" do
       code = """
       defmodule MyLocalModule do